1. Field of the Invention
The present invention relates to a process for selective dimerization of propylene principally into branched dimers. More particularly, it relates to a process in which propylene is brought into contact with a catalytic composition resulting from at least partly dissolving a nickel compound mixed or complexed with a tertiary phosphine carrying a functional group in a medium resulting from mixing at least one quaternary ammonium halide and/or at least one quaternary phosphonium halide, at least one aluminum halide and optionally at least one organometallic aluminum compound.
2. Description of the Prior Art
French Patent No. 2,611,700 describes the use of liquids of ionic nature formed from aluminum halides and quaternary ammonium halides as solvents for organometallic nickel complexes to catalyze olefin dimerization. The use of such media, which are not miscible with aliphatic hydrocarbons, in particular with the products from olefin dimerization, enables homogeneous catalysts to be used more effectively. French Patent No. 2,659,871 describes a liquid composition with an ionic nature resulting from bringing quaternary ammonium halides and/or quaternary phosphonium halides into contact with alkylaluminum dihalides and optionally also an aluminum trihalide. That same patent describes the use of such media as solvents for transition metal complexes, in particular nickel complexes containing no nickel-carbon bond, which are transformed into catalysts for olefin oligomerization. In the present text, such media will henceforth be termed xe2x80x9cmolten saltsxe2x80x9d, as they are liquid at moderate temperatures.
During those studies, it was shown that the most active and most stable nickel catalysts are obtained in xe2x80x9cmolten saltsxe2x80x9d constituted by one molar equivalent of an ammonium halide and/or a phosphonium halide with one equivalent and more of an aluminum trihalide, and optionally any quantity of an alkyl aluminum dihalide. That formulation has been shown to be particularly interesting as nickel complexes dissolved in it have high catalytic activity.
Further, it has been shown that under the conditions described in French Patent No. 2,611,700? the xe2x80x9cphosphine effectxe2x80x9d, as described by G. Wilke et al., in Ind. Eng. Chem., 1970, 62, No. 12, p. 34, and in U. K. Patent No. 1,058,680, which reports the influence of substituents carried by the phosphorus atom on the mode of enchainment of propylene molecules during catalytic dimerization by nickel, rapidly disappears over time. That unexplained phenomenon has deleterious consequences since it does not produce the desired selectivities.
French Patent No. 2,710,280 shows that adding an aromatic hydrocarbon to a xe2x80x9cmolten saltxe2x80x9d can overcome this problem and result in catalysts with high activity which are more stable and which have a high selectivity for the most highly branched isomers. However, the aromatic hydrocarbon is continuously extracted in the organic phase constituted by the products, which implies that it must be separated and recycled to the reactor.
It has now been discovered that the use of a tertiary phosphine carrying a functional group or a nickel complex formed with a tertiary phosphine that is soluble in the xe2x80x9cmolten saltxe2x80x9d results in catalysts for which the selectivity for the most highly branched isomers is high and stable over time and for which the activity is high.
The invention provides a process for selective dimerization of propylene into branched dimers in which a catalytic composition comprising at least one nickel compound mixed or complexed with at least one tertiary phosphine carrying a functional group is used, at least partly dissolved in a non aqueous medium with an ionic nature (xe2x80x9cmolten saltxe2x80x9d type medium), resulting from bringing at least one aluminum halide (product B) into contact with at least one quaternary ammonium halide and/or at least one quaternary phosphonium halide (product A), the xe2x80x9cmolten saltxe2x80x9d type medium possibly further comprising at least one organometallic aluminum compound (product C).
Thus the xe2x80x9cmolten saltxe2x80x9d type medium, in which the nickel compound mixed or complexed with at least one tertiary phosphine carrying a functional group or at least one phosphite carrying a function croup is dissolved, is constituted by mixing:
a) at least one quaternary ammonium and/or quaternary phosphonium halide, more particularly a chloride and/or bromide (product A);
b) at least one aluminum halide (product B); and
c) optionally, at least one organometallic aluminum compound (product C).
Preferred quaternary ammonium and/or phosphonium halides that can be used within the context of the invention (product A), are:
those with general formula NR1R2R3R4X (with the exception of NH4X), PR1R2R3R4X, R1R2Nxe2x95x90CR3R4X or R1R2Pxe2x95x90CR3R4X, where X represents Cl or Br and R1, R2, R3 and R4, which may be identical or different, each represent hydrogen or a hydrocarbyl residue containing 1 to 12 carbon atoms, for example saturated or unsaturated alkyl, cycloalkyl or aromatic groups, aryl groups or aralkyl groups, containing 1 to 12 carbon atoms, it being understood that preferably, only one of substituents R1, R2, R3 and R4 represents hydrogen;
or one of the following general formulae: 
xe2x80x83where the nitrogen-containing or phosphorus-containing heterocycles containing 1, 2 or 3 nitrogen and/or phosphorus atoms are constituted by 4 to 10 atoms and X, R1 and R2 are defined as above.
Examples which can be cited are tetrabutylphosphonium chloride, N-butyl pyridinium chloride, ethylpyridinium bromide, 3-butyl- l-methyl imidazolium chloride, diethylpyrazolium chloride, pyridinium hydrochloride, trimethylphenyl ammonium chloride and 3-ethyl-1-methyl imidazolium chloride. These salts can be used alone or as a mixture.
The aluminum halides used as products B of the invention are essentially aluminum chloride and bromide.
The organometallic aluminum compounds used as optional products C of the invention have general formula AlRxX3-x in which R is a linear or branched alkyl residue containing 2 to 8 carbon atoms, X is chlorine or bromine and the value of x is 1, 2 or 3. Examples of organometallic aluminum compounds that can be used are isobutylaluminum sesquichloride, ethylaluminum sesquichloride, dichloroisobutyl aluminum, dichloroethyl aluminum and chlorodiethyl aluminum.
The components of the xe2x80x9cmolten saltsxe2x80x9d as defined above are generally used in A:B mole ratios of 1:0.5 to 1:3, preferably 1:1 to 1:2; product C is used in a mole ratio of at most 100:1 with product B, preferably 0.005:1 to 10:1. However, the components and their proportions must be such that the mixture is liquid at the temperature at which the nickel compound and the functionalized tertiary phosphine are introduced, although the catalytic dimerization reaction can be carried out at a temperature which is lower than or higher than the fusion temperature of the catalytic composition.
Examples of nickel compounds used in the catalytic compositions of the invention are the chloride, bromide, sulfate, carboxylates (for example the 2-ethylhexanoate), phenates and acetyl acetonate. It is also possible to use organometallic nickel complexes which may or may not contain phosphines. These nickel complexes are used as a mixture with a functionalized tertiary phosphine. It is also possible to use nickel complexes that are already complexed with a tertiary phosphine carrying a function.
The functional phosphines used as a mixture with (or to complex) the nickel compounds of the invention have general formulae PRxe2x80x21Rxe2x80x22Rxe2x80x23 and Rxe2x80x21Rxe2x80x22Pxe2x80x94Rxe2x80x2xe2x80x94PRxe2x80x21Rxe2x80x22, where Rxe2x80x21, Rxe2x80x22 and Rxe2x80x23, which may be identical or different, are alkyl, cycloalkyl, aryl or aralkyl radicals containing 1 to 10 carbon atoms at least one of which carries a functional group such as an amine, a cyclic amine, a nitrogen-containing heterocycle, an ester, an acid, an alcohol, a quaternary ammonium, a quaternary phosphonium, a sulfonium, a sulfonate or a phosphonate and Rxe2x80x2 is a divalent aliphatic residue containing 1 to 6 carbon atoms.
The functional phosphines can be selected from compounds containing pyridine or imidazole substituents or their quaternized derivatives containing pyridinium or imidazolium substituents that satisfy formulae 1 to 7 defined below.
Examples of functional phosphines carrying a pyridine substituent are 2-dicyclopentyl-phosphinoethyl-4-pyridine with formula (1), 2-dicyclopentylphosphinoethyl-2-pyridine with formula (2), 2-diisobutylphosphinoethyl-4-pyridine with formula (1b), 2-diisopropylphosphinoethyl-4-pyridine with formula (4) and their quaternization derivatives with formula (3), where R is an alkyl group containing 1 to 10 carbon atoms and X is a weakly coordinating anion. Examples of weakly coordinating anions which can be cited are tetrafluoroborate, hexafluorophosphate, tetrachloroaluminate, hexafluoroantimonate, carboxylate anions such as acetate, trifluoroacetate, trifluorosulfonate, and the anions N(CF3SO2)2xe2x88x92 and C(CF3SO2)3xe2x88x92. Examples of quaternization derivatives are 2-dicyclopentylphosphinoethyl-N-ethyl pyridinium tetrafluoroborate with formula (3a), or 2-dicyclopentylphosphinoethyl-N-ethyl pyridinium chloride with formula (3b).
Examples of functional phosphines carrying an imidazole substituent which can be cited are 2-dicyclopentylphosphinoethyl-N-imidazole with formula (5), 2-diisopropylphosphinoethyl-N-imidazole with formula (7), 2-diisobutylphosphinoethyl-N-imidazole with formula (7b) and their quaterization derivatives with formula (6), where R is an alkyl group containing 1 to 10 carbon atoms and X is a weakly coordinating anion (as defined above), such as 2-dicyclopentyl-phosphinoethyl-1-methyl imidazolium tetrafluoroborate with formula (6a). 
As examples of nickel compounds that can be used to constitute the catalytic compositions of the invention, can be cited the complex complexes [NiCl2, 1.5P(2-dicyclopentylethyl-4-pyridine)]2, [NiCl2, 2P(2-dicyclopentylethyl-N-ethyl pyridinium tetrafluoroborate)], [Ni2Cl4, (2-dicyclopentylphosphinoethyl-N-ethyl pyridinium tetrafluoroborate)3, 1.5 CH2Cl2], NiCl2, 2 pyridine mixed with at least one equivalent of functionalized tertiary phosphine or functionalized phosphite, nickel chloride mixed with at least one equivalent of 2-dicyclopentylphosphinoethyl-4-pyridine, nickel acetate mixed with at least one equivalent of 2-dicyclopentylphosphinoethyl-4-pyridine, nickel (2-ethyl hexanoate) octoate mixed with at least one equivalent of 2-dicyclopentylphosphinoethyl-4-pyridine and 2-dicyclopentylphosphinoethyl-4-pyridine xcfx80-allyl nickel chloride.
The compounds forming part of the catalytic composition of the invention can be mixed in any order. The mixture can be produced by simply bringing them into contact followed by agitation until a homogeneous liquid is formed. This mixture can be produced outside the dimerization or oligomerization reactor or, as is preferable, in the reactor.
The propylene which undergoes the selective dimerization of the invention can be used pure or diluted in an alkane, such as those found in C3 cuts from oil refining processes, such as catalytic cracking or steam cracking.
The catalytic propylene dimerization reaction can be carried out in a closed system, in a semi-open system or continuously, with one or more reaction stages. Vigorous agitation must be carried out to ensure good contact between the reactant or reactants and the catalytic mixture. The reaction temperature can be from xe2x88x9240xc2x0 C. to +70xc2x0 C., preferably xe2x88x9220xc2x0 C. to +50xc2x0 C. It is possible to operate above or below the fusion temperature of the medium, the dispersed solid state not being a limitation to the proper conduct of the reaction. The heat engendered in the reaction can be eliminated using any means known to the skilled person. The pressure can be from atmospheric pressure to 20 MPa, preferably atmospheric pressure to 5 MPa. The reaction products and the reactant or reactants that has/have not reacted are separated from the catalytic system simply by decanting, then fractionation.
The entire disclosure of all applications, patents and publications, cited above and below, and of corresponding French application 00/01511, filed Feb. 4, 2000, are hereby incorporated by reference.
The following examples illustrate the invention without limiting its scope.